Diethylstilbesterol (or diethylstilbetrol; DES) was a drug prescribed to women from 1938 through 1971 to prevent miscarriages in high-risk pregnancies. However, acting as a potent estrogen, exposed female fetuses had an increased risk to developing abnormal reproductive tracts and cancer.

In the mid-1970s, numerous cases of vaginal adenocarcinoma in women between the ages of 16 and 20 were linked to their mothers taking diethylstilbesterol early during the pregnancy. With one out of every 1,000 pregnant women being prescribed DES, millions of people were exposed, and about 75 percent of their daughters developed vaginal and cervical cancer as well as anomalies of the genital tract. Known as human DES syndrome, these women have a deformed Müllerian tube with loss of the boundary between the uterus and the oviduct, and there is often a failure to form a single cervical canal. Without diethylstilbesterol, estrogen and progesterone normally influence the development of the Müllerian duct and cervix. Experiments with mice demonstrated that injected DES repressed the expression of the Hoxa-10 gene, resulting in problems with the developing mesenchyme of the duct. In humans, males exposed during gestation also displayed abnormal genitalia.

Marketed originally under the trade name desPlex® and now stiphostrol® as a synthetic, nonsteroid with the properties of estrogen, diethylstilbesterol was also prescribed to women for treating menstrual problems. Being teratogenic, DES was banned by the United States Food and Drug Administration (FDA) in 1979 for use in the livestock industry as a growth promoter. The performance and unsolicited problems of diethylstilbesterol are strongly tied to its molecular structure, which mimics estrogens. However, for women with inoperable, post-menopausal breast cancer, and for treatment of inoperable metastatic prostatic carcinoma, diethylstilbesterol has regained some respect in the growing field of palliative medicine. In this role, DES acts through competition with estrogenic and androgenic compounds, and binds to tumor cells, thus inhibiting further growth. It is ironic that DES is labeled as a known carcinogen, being an estrogen agonist, but at the same time is now used as an anticancer agent. Some physicians prescribe diethylstilbesterol as a morning after pill to prevent fertilized egg implantation in the uterus. DES has been detected in some drinking water supplies at concentrations of 0.11 to 0.26 nanograms per liter, and meat consumers may have been exposed to levels of approximately 10 parts per billion in beef and sheep prior to 1979.

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